1. Field of the Invention
This invention pertains to a high speed common mode logic (CML) circuit, and more particularly to a CML circuit having an active MOS load operating exclusively in the triode region to provide improved performance characteristics.
2. Description of the Related Art
The basic building block of a digital integrated circuit is a logic circuit or logic gate such as an inverter, AND gate, OR gate, NAND gate, NOR gate, XOR gate, etc. With the advent of the integrated circuit (IC), the discrete components (diodes, transistors, and resistors) which make up logic gates are fabricated on a small semiconductor crystal, sometimes called a chip. The various gates are interconnected inside the chip to form the required circuit. Digital ICs are often categorized according to their circuit complexity as determined by the number of logic gates on a single chip. Depending on the number of internal gates, which may range from only a few up to thousands, each chip is customarily referred to as being either a small-, medium-, large-, or very large-scale integration device.
Digital ICs are classified not only by their logical operation and complexity, but also by the specific circuit technology which they employ. The circuit technology is referred to as a logic family, and each family has a set of basic circuits upon which more complex digital circuits and components are developed. The basic circuits may include, for example a NAND gate, a NOR gate, an inverter, etc. Many different logic families of digital integrated circuits are commercially available. Some of the more important logic families include TTL (transistor--transistor logic), ECL (emitter-coupled logic), MOS (metal-oxide semiconductor), and CMOS (complementary metal-oxide semiconductor). Although each of these logic families has its own advantages and disadvantages, and the choice of which logic family to use in a particular application depends on various factors such as speed, cost, noise immunity, power consumption, etc., CMOS has emerged as the most popular and versatile logic family.
CMOS digital integrated circuits are useful in many different kinds of applications and are fabricated at all levels of integration, from SSI to VLSI. CMOS integrated circuits can operate at high speeds with relatively low power consumption and are more tolerant of power supply fluctuations than devices made from circuits in other families. However, with the demand for faster and quieter integrated circuits increasing, there is a need for CMOS logic circuits which operate at even higher speeds with even less power supply fluctuation than those currently available.